In a motorcycle of the shaft drive type, it is conventional practice for a propeller shaft case to act as one element of a rear arm to which the rear wheel is mounted. In this situation, the frame of the motorcycle is bifurcated whereby to interpose the front end of the rear arm inbetween. It is a requirement that the universal joint of the propeller shaft be disposed substantially on the center line of the bearings that connect the rear arm to the frame. This is because the relative positions between the propeller shaft and the rear arm must be invariable when the rear wheel moves up and down and the rear arm is rocked back and forth. As in a chain drive system, a supporting member extending between the right and left frames for supporting the pivot shaft of the rear arm cannot be provided because it would cause interference with the propeller shaft. As a result, because the supporting member has to be divided into halves at the right and left sides of the frame and still has to be able to support the rear arm in its divided form, a very high strength is required to be provided by the supporting structure.
On the other hand, a motorcycle must be narrow as possible so as to reduce its sidewise (widthwise) protrusion. However, the position of the propeller shaft in the widthwise dimension of the motorcycle body is determined by its relationship with the rear wheel. Therefore, according to the conventional construction in which both ends of the rear arm are interposed between the frame halves, the width of the motorcycle is accordingly increased when the mounting portion of the frame is located outside (laterally away from the forward axis of the frame) of the propeller shaft, so that the objective of narrowing the motorcycle is frustrated.